The Spanish region of Catalonia is planning to hold an independence referendum on Sunday, but the Madrid government has called the vote illegal and vowed to block it. Here’s a rundown of what to expect from the ballot—and the impact it might have.

What can be expected from the referendum?

The government of Catalonia, a wealthy region in northeastern Spain with a distinct history and language, is forging ahead with an independence referendum, even though Spain’s constitutional court has suspended the vote and the central government...